1. Field of the Invention
The present invention relates to a plasma X-ray source for generating high output and highly stable soft X-rays to be used in an exposure apparatus for replicating a fine pattern to be used in the fabrication of semiconductor integrated circuits, and an X-ray lithography method utilizing the X-ray source.
2. Description of the Prior Art
One of the lithography processes which play a very important role in the fabrication of integrated circuits is an X-ray lithography. So far an electron beam impact source in which X-rays are generated by the bombardment of an electron beam against a target consisting of Al, Cu, Mo, Si, Pd or the like has been used as a soft X-ray source of an X-ray exposure apparatus. However, this source has the problem that the X-ray generating efficiency is as low as 0.01%, so that high output X-rays cannot be obtained and consequently the pattern replication yield is low.
On the other hand, a plasma X-ray source utilizing a high density plasma has a high X-ray generating efficiency, so that it is expected that high output X-rays are obtained. The efficiency of the conversion of the energy applied to a plasma X-ray source into soft X-rays is higher than 1%, so that a high efficiency hundred times as high as the efficiency of an electron beam impact X-ray source is expected.
In the case of a plasma X-ray source, a plasma is produced by a discharge. A large current of the order of hundreds kA flows through the plasma so that the plasma is caused to pinch by its own magnetic field produced by the current and the electromagnetic action of the plasma to produce a high temperature and high density plasma. X-rays are emitted from the high temperature and high density plasma. Since the plasma is produced by flowing a large current as described above, there arise the problems of stability in X-ray intensity, electrode consumption and damage to an X-ray extraction window by a plasma. As a result, it is extremely difficult in practice to use the plasma X-ray source as an X-ray exposure source.
Gas injection type plasma X-ray sources are disclosed in the following papers:
(1) "X-ray lithography using a pulsed plasma source", Pearlman et al., J. Vac. Sci. Technol., 19(4), Nov./Dec. 1981, pp. 1190-1193;
(2) "Evaluation of the gas puff Z pinch as an X-ray lithography and microscopy source", J. Bailey et al., Appl. Phys. Lett. 40(1), Jan. 1, 1982, pp. 33-35; and
(3) "Imploding argon plasma experiments", C. Stallings et al., Appl. Phys. Lett. 35(7), 1979, pp. 524-256.
FIG. 1 shows a gas injection type plasma X-ray source which is disclosed in these papers. In FIG. 1, reference numeral 1 denotes a vacuum vessel; 7, a fast acting puff valve; 13, a capacitor; 17, a gas injection electrode; 18, a mesh electrode; 21, a switch; 26, X-rays produced; 28, an X-ray extraction window of a Be film; 29, an X-ray mask; 30, a wafer; 43, a gas plenum provided in the fast acting puff valve; 45, a piston; 61, a gas jet; 63, a pinched plasma; and 64, a stream or group of charged particles. In a gas injection discharge method, the gas stored in the gas plenum 43 is instantaneously forced into the space between the electrodes 17 and 18 which are in opposed relationship with each other in the vacuum vessel 1 by driving the piston 45 of the fast acting puff valve 7 at a high speed, whereby the gas jet 61 is formed between the electrodes 17 and 18. Thereafter, the switch 21 is closed so that a voltage is applied between the electrodes 17 and 18 from the capacitor 13, which is charged. As a result, the gas jet 61 is ionized by a discharge and is converged by a current, so that a plasma is pinched toward the center. Thus, the high temperature and high density plasma 63 is produced.
In the gas injection discharge method, it is necessary that the rising slope of the injected gas be formed as steeply as possible, so that when the gas jet is formed between the electrodes, it has a gas density adapted to cause a discharge before the gas jet is diffused. The time variation in the flow rate Q of the gas which flows when the piston 40 of the fast acting puff valve is opened is expressed by the following equation: ##EQU1## where Po: the gas pressure of the plenum in the fast acting puff valve 7;
D: conductance of the passage through which the gas flows; and PA1 l: distance of the gas passage.
The above equation shows that in order to obtain a steep gas profile, by increasing the volume of the flowing gas as much as possible, the high gas pressure Po is adopted usually. Therefore, in the prior art, the fast acting puff valve usually introduces the gas at a high pressure of about 5 atm. so that the gas injection velocity is increased.
Meanwhile, in the prior art, a gas is injected between the electrodes by operating the fast acting gas valve with a high plenum gas pressure, as described above, so that the gas density between the electrodes reaches a high value in the range of 1.times.10.sup.20 -1.times.10.sup.22 cm.sup.-3. Moreover, it takes about 0.1 ms to open or close the fast acting puff valve. Therefore, the gas density between the electrodes is increased after the gas jet is formed and discharged between the electrodes, and then the plasma 44 is pinched. As a result, the gas density is increased during the discharge caused by the current flowing after the plasma has been pinched. Therefore, a high pressure arc discharge, which is one of the characteristics of a discharge at a high gas pressure, occurs so that the electrodes are locally heated. As a consequence, the melting of electrodes is accelerated and the electrodes are consumed so that the materials of the electrodes contaminate the inner walls of the vessel. Furthermore, high energy electrons and ions and high temperature gases which are produced by the discharge are increased.
When such a high temperature and high density plasma is used as an X-ray exposure source, electrode consumption degrades discharge reproducibility and the stability of X-ray emission. Furthermore, the breakdown voltage of an insulator is decreased because of the adhesion of electrode materials to the surfaces of the insulator to which a high voltage is applied. When the plasma X-ray source is used as an X-ray exposure source, the transmissivity of X-rays through the X-ray extraction window 28 is decreased because of the deposition of the electrode materials on the X-ray window. As a result, a continuous X-ray exposure is impossible. Moreover, because the gas density between the electrodes reaches a high value, the high energy charged particles and high temperature gases impinge against the X-ray extraction window, so that the X-ray extraction window 28 is damaged.
Especially, when the plasma 63 is produced by the gas jet 61 along the axis of the electrodes as shown in FIG. 1, a large amount of high energy charged particles such as ions and electrons are emitted in the direction of the axis of the plasma 63. As a result, even when the X-ray extraction window 28 is located in the direction of the axis of the plasma, it is seriously damaged, so that it is impossible to make an exposure. Therefore, as shown in FIG. 1, the X-ray window 28, the X-ray mask 29 and the wafer 30 are located in the radial direction of the pinched plasma 63 and an exposure is made in vacuum.
FIG. 2 shows an X-ray pin-hole picture photographed in the radial direction of the X-ray source through a Be film which is disposed at the position of pattern replication. When a proximity exposure method is employed, the X-ray source becomes linear so that a viewing angle is increased and the replicated pattern is largely blurred. As a consequence, it is impossible to replicate a fine pattern. In view of this, when a conventional gas injection type plasma X-ray source is used, it is only possible to make an exposure in a radial direction of the X-ray source, so that the X-ray source is not suitable as an X-ray source for the exposure of a fine pattern.
In addition, when a gas is injected at a high pressure, a large volume of gas is introduced, so that it takes a long time to exhaust the gas in the vacuum vessel. As a result, in the gas injection type X-ray source, it is impossible to repeat discharges at a high repetition rate.
Furthermore, when an X-ray source is used for exposure, the charged particles which impinge against the X-ray extraction window must be reduced as much as possible. So far, no satisfactory method has been proposed to overcome this problem.